Galactic Patrol by E.E. Smith

Gradually and so insidiously that the Velantian's dire warnings might as well never have been uttered, the scene changed. Or rather, the scene itself did not change, but the observers' perception of it slowly underwent such a radical transformation that it was in no sense the same scene it had been a few minutes before; and they felt almost abjectly apologetic as they realized how unjust their previous ideas had been.

For the cavern was not a torture chamber, as they had supposed. It was, in reality, a hospital, and the beings they had thought victims of brutalities unspeakable were, in reality, patients undergoing treatments and operations for various ills. In proof whereof the patients -- who should have been dead by this time were the early ideas well-founded -- were now being released from the screenlike operating theater. And not only was each one completely whole and sound in body, but he was also possessed of a mental clarity, power, and grasp undreamed of before his hospitalization and treatment by Delgon's super surgeons!